In a polymer or copolymer synthesis, achieving a product with a desired molecular weight and a narrow weight distribution, or polydispersity, requires a controlled process. Polymers with a narrow molecular weight distribution can exhibit substantially different behavior and properties than polymers prepared by conventional means. Living polymerizations provide the maximum degree of control for the synthesis of polymers with predictable well-defined structures. The characteristics of a living polymerization include: polymerization proceeding until all monomer is consumed, number average molecular weight as a linear function of conversion, molecular weight control by the stoichiometry of the reaction, and block copolymer preparation by sequential monomer addition.
It has been stated that living polymerization to give polymers of low molecular weight distribution requires the absence of chain transfer and termination reactions. In a living polymerization, the only “allowed” elementary reactions are initiation and propagation, which take place uniformly with respect to all growing polymer chains. However, it has also been shown that if the chain transfer process is reversible, polymerization can still possess most of the characteristics of living polymerization.
It has been found that the reversible addition-fragmentation chain transfer (RAFT) process suppresses termination reactions through the addition of a suitable thiocarbonylthio compound, also known as a dithioester, to an otherwise conventional free radical polymerization. Control in such a RAFT process is thought to be achieved through a degenerative chain transfer mechanism in which a propagating radical reacts with the thiocarbonylthio compound to produce an intermediate radical species. This process decreases the number of free radicals available for termination reactions that require two free radicals.
Although RAFT polymerizations have been demonstrated to work under a variety of conditions, further research is required to demonstrate the effectiveness of RAFT polymerizations in aqueous solvent systems. Specifically, there is a need to develop dithioester chain transfer agents that are both soluble and stable in water. Also, there is a need to develop dithioesters that are tailored to the monomer being polymerized.